add new section in HMM tutorial

[JasonPekos]

This adds a section to the HMMs tutorial that has a much faster and more concise version of the model (four lines!) using HiddenMarkovModels.jl.

There's a subsection after that which shows how to recover the state estimates with the Viterbi algorithm.

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This closes:

• Add an implementation of forward-backward algorithm for HMM models #330

[github-actions]

Preview the changes: https://turinglang.org/docs/pr-previews/508
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[yebai]

Thanks @JasonPekos. It looks good!

[penelopeysm]

Thanks @JasonPekos for putting this together! I've just gone ahead and made my suggested changes directly --- and also updated a few lines so that it would be compatible with current Turing versions, so will merge now :)